The enzymatic activities of acetyl CoA carboxylase and fatty acid synthetase have been determined in the normal lactating mammary glands of rats and have been compared with the activities obtained from the transplantable mammary tumors, 13762 and R3230 AC. The levels of these activities in the mammary tumors are approximately 5-20% of those noted in the lactating gland. Furthermore, it has been observed that the faster growing tumor (13762) possesses both the acetyl CoA carboxylase and fatty acid synthetase activities which are 2 to 3 times higher than found in the slow growing tumor R3230 AC. The activity of acetyl CoA carboxylase is inhibited when the 700g supernatant of the normal lactating mammary gland is treated with either ATP or dibutyryl cAMP prior to assay. The 100,000g supernatant of the normal gland is susceptible to inhibition by only cAMP treatment, however. Qualitatively similar data have been obtained on the inhibition of acetyl CoA carboxylase present in the 700g supernatants of tumors 13762 and R3230AC. The susceptibility of acetyl CoA carboxylase present in the high speed supernatant of tumor 13762 is similar to that of a comparable preparation obtained from the normal mammary gland. However, the enzyme present in the 100,000g supernatant of tumor R3230 AC homogenate is capable of inhibition by both ATP and cAMP. Acetyl CoA carboxylase activity of these tumors appears to undergo inhibition more readily than the enzyme obtained from the normal lactating mammary gland.